Consumer beverages such as juice, sports drinks or similar uncarbonated or "flat" beverages can be packed in, for example, plastic bottles of polyester (PET). This type of bottle is normally produced from injection moulded blanks or preforms which are manufactured from a central source and then supplied to bottle blowing machines. In the bottle blowing machine, the preform is heated and mechanically stretched in its longitudinal direction, whereafter it is subjected to a pressure difference and is inflated to abutment against the inside of a twin body mould.
A similar type of blown plastic bottle is also manufactured in that a prefabricated hollow preform is given the desired container form by heating and inflation in an enclosing or envelope mould. However, instead of using as starting material an injection moulded preform, the preform is manufactured from web-shaped thermoplastic material, e.g. polyester (PET), or high density polyethylene (HDPE), in which instance one web is formed into a cylinder while the other web is used for producing bottom sections which are fixedly welded to the cylinder so that a preform or parison is created. By manufacturing the preform from web-shaped material, it is possible to minimise material consumption at the same time as utilising in a simple manner laminated material, including, for example, layers of gas barrier material.
The parts thus formed from two material webs, i.e. a tubular casing portion and a substantially circular bottom plate, can be connected to one another in a liquid-tight welding joint with the aid of heat and pressure. In such instance, the conventional wisdom is to make use of, for example, hot air nozzles which heat the edge regions of both of the parts to the softening or fusion temperature of the plastic material, whereafter the parts are brought together and urged together until such time as the material has fused and stabilised. In this case, a certain amount of heat is supplied which, for various reasons such as the working environment and energy consumption, should be cut to a minimum. Excessive heating of the edge regions of both of the parts also involves a risk of permanent thermal deformation of adjacent areas at the same time as the cooling operation takes longer, which reduces production output rates. Another method of heating and sealing the two parts to one another has therefore been tested, namely the use of an ultrasound horn with a substantially annular work surface which, with the aid of a substantially mandrel-like counter abutment disposed in the sleeve, presses together the edge regions of both the bottom portion and the casing portion under the simultaneous supply of energy. Theoretically, this welding method should be rapid and feasible with reduced energy consumption, but practical trials and experiments have shown that the ultrasonic energy is conducted to and concentrated at the central region of the bottom plate, which, as a result, is readily deformed and breaks in connection with the ultrasound welding operation. In a circular bottom plate, a central hole of a few millimetres in diameter passing through the bottom plate typically arises in connection with the welding operation. It has proved in the trials conducted that this problem generally occurs in ultrasound welding of bottom plates or circular blanks and it has hitherto not proved possible using prior art methods wholly to obviate this drawback. Since a not inconsiderable proportion of the ultrasonic energy is led to the central region of the bottom plate, the heating of the actual welding site is correspondingly reduced, i.e. the circular bottom weld joint, with the result that incomplete or weakened welding joints are obtained. There is thus a general need in the art to realise a method of making possible ultrasound welding of an annular area on circular material blanks without the central region of the circular blanks being subjected to damage.